Progress in the science of measuring ground motion from distant earthquakes dates primarily to the last one hundred years. Key developments were the invention by La Coste and Romberg of a long-period vertical seismograph in 1934, and the introduction of active electronic sensing and feedback pioneered by Block and Moore in the 1960's, culminating in what is still the state-of-the-art long-period leaf-spring seismometer of Wielandt and Streckeisen in 1982. These seismometers are constructed with large inertial masses supported by soft helical springs or curved leaf-springs and force-balance feedback systems. Improvements include using springs of constant modulus materials, advanced analog-to-digital converters, and digital filters. The primary output is an analog signal that is related to ground velocity, which can be converted to ground acceleration by differentiation. Typical sensitivities to slow ground acceleration are measured in nano-g's or better. To achieve the high sensitivity, the applied acceleration is usually limited to only about 1% of the earth's gravity. Thus the full scale range is limited and the output is clipped if the earthquake is strong.
On the other hand, the science of measuring ground acceleration directly, for instance with force-balance accelerometers, has resulted in devices that have a much larger acceleration full scale and are particularly sensitive at short time intervals. These devices are commonly called strong-motion sensors. Their shortcoming is that they are not very sensitive to weak long-period vertical ground acceleration.
Another disadvantage of conventional long-period and strong-motion seismometers is the analog output of the sensor that is converted into a digital signal by an analog-to-digital converter with limited numeric dynamic range and poor long-term stability. An intrinsically digital seismic and gravity sensor with frequency output that can be measured in the time domain in relation to a very precise clock standard provides very high short-term resolution and the highest long-term stability.
Therefore, a need exists for high-resolution, inherently digital seismic and gravity sensors that measure accelerations directly, particularly sensors that are compact in size, use low power, have low temperature sensitivities, and use non-magnetic materials.
A number of force-sensitive resonators are described in the prior art. Single vibrating beam force sensors are described in U.S. Pat. Nos. 3,470,400, 3,479,536, 4,445,065, 4,656,383, 4,658,174, 4,658,175, 4,743,790, 4,980,598, 5,109,175, and 5,596,145. Double vibrating beam force sensors referred to as Double-Ended Tuning Forks (“DETF”) are described in U.S. Pat. Nos. 2,854,581, 3,148,289, 3,238,789, 4,215,570, 4,372,173, 4,415,827, 4,469,979, 4,531,073, 4,757,228, and 4,912,990. In these devices, the change in frequency of oscillation of the resonant force sensors is a measure of changes in the applied force.
Single-axis accelerometers employing resonator beams are disclosed in U.S. Pat. Nos. 2,984,111, 3,190,129, 3,238,789, 3,440,888, 3,465,597, 4,091,679, 4,479,385, 4,980,598, 5,109,175, 5,170,665, 5,334,901, and 5,596,145. In general, the devices disclosed in these patents are open-loop sensors without servo feedback, consisting of an inertial mass that exerts a force on the resonator under acceleration along the sensitive axis. The inertial mass is usually guided by a suspension system or flexures. None of these devices reaches the sensitivity of state-of-the-art long-period seismometers, as the full scale is always in excess of the earth's gravitational acceleration and the dynamic range is not high enough to reach a sensitivity of nano-g's or better. Triaxial accelerometers employing force-sensitive resonators are disclosed in U.S. Pat. No. 6,826,960 and in U.S. Pat. No. 7,178,401.